The transduction of light-energy into a receptor potential in the rod photoreceptor of the vertebrate retina is initiated with the absorption of light by the membrane bound visual pigment Rhodopsin. Upon absorption of light, Rhodopsin undergoes a series of conformational changes which trigger a change in the ionic conductance of the rod plasma membrane. The project proposed herein investigates the mechanism which couples the excitation of Rhodopsin to changes in plasma membrane conductance by studying the effects of light on ionic permeabilities in the rhopdopsin-containing disc membranes of the rod cell. Ionic permeabilities are studied by measuring disc membrane potentials with a method that records the voltage-dependent distribution of hydrophobic ions between the inside and outside of the discs. The project further investigates whether Rhodopsin is directly involved in any light- dependent permeability changes of the disc membrane, and if so by what mechanism, by studying the ionic permeability characteristics of model membranes into which rhodopsin is incorporated. The ionic permeabilities of the model membranes are investigated with electrophysiological methods. Finally, the results obtained in the model system are substantiated by investigating the physical-chemical characteristics of Rhodopsin in the model membranes with electrical and spectrophotometric methods, and comparing these characteristics in the model system with those in the disc membrane.